Improvements in optical amplifiers have vastly enhanced the use of optical communication systems by increasing both data rates and the distances over which optical signals are transmitted. One significant advancement in such amplifiers is the development of optical amplifiers based on an optic fiber doped with a rare-earth element. This type of amplifier eliminates the need for complicated regenerator or "repeater" devices used in conventional systems to regenerate attenuated signals. Conventional repeaters require complex electronics to convert light into electric signals, amplify the signal, recover the data from the amplified signal, and then transform it back into light. In contrast, doped-fiber optical amplifiers do not interrupt the light signal, but merely add energy to it as described below. The components in the optical amplifier system are comparatively simple.
FIG. 1 illustrates a block diagram of a prior art optical amplifier designated generally at 10. The block diagram is a simplified illustration of commercially available amplifiers, such as the FiberGain.TM., Module available from Corning Incorporated of Corning, N.Y., and identified as part number CL-10. Amplifier 10 includes an optical fiber 12 which is doped with a rare-earth element. In the preferred embodiment, the dopant is erbium. Other elements, such as neodymium, also have been used as dopants for the fiber, but erbium remains as the most prominent and successful. Optical fiber 12 provides an input 14 for receiving a light input signal, L.sub.IN, and an output 16 for providing an amplified light output signal, L.sub.OUT1. While not shown, optical fiber 12 is typically wound around a spool or like device.
Amplifier 10 further includes a light sourcing device which is typically a laser diode 18. While not shown, laser diode 18 is commonly housed in a small metallic case. Laser diode 18 couples power to the amplifier by "pumping" energy into optical fiber 12 and, hence, is also known as a pump laser diode. Specifically, the light provided by laser diode 18 is absorbed by the erbium atoms in fiber 12, pumping those atoms to a high-energy level. When a weakened L.sub.IN signal enters fiber 12, the excited erbium atoms transfer their energy to the signal in a process known as stimulated emission. As a result, the fiber 12 provides the amplified light output signal, L.sub.OUT1. The anode of laser diode 18 provides an input 20 for receiving an amplification control current, I.sub.c. For ease of illustration, the cathode of laser diode 18 is shown as grounded. It should be understood, however, that alternative configurations may be implemented for activating and deactivating laser diode 18.
Amplifier 10 also includes a pump power detector 22, which is typically a photodiode. Power detector 22 is proximate the sourcing laser diode 18 and, hence, provides an electrical signal, I.sub.INT1, directly proportional to the light intensity L.sub.PLD of pump laser diode 18. Signal I.sub.INT1 is detectable at output 24 of amplifier 10. In commercially available amplifiers, power detector 22 is often referred to as a "rear beam detector" due to its locational relationship to laser diode 18. Specifically, a small portion of the light emitted by laser diode 18 is reflected "rearwardly" to the detector, thereby giving the detector its name. As known in the art, the photodiode converts the light to an electrical signal (i.e., I.sub.INT1) indicative of the intensity of the detected light.
While amplifier 10 of FIG. 1 provides numerous advantages over repeaters, developmental efforts continue in an attempt to increase optical system performance, including device reliability. For example, it is known in the art to include a feedback circuit which adjusts I.sub.c to maintain a constant light output signal, L.sub.OUT1. Thus, as L.sub.IN changes in intensity or wavelength, I.sub.c is altered to maintain L.sub.OUT, at a fixed level. When L.sub.IN falls below a certain level, or is removed completely, the feedback system would try to greatly increase the magnitude of I.sub.c. However, above a certain level of optical output power, diode 18 will be damaged. Therefore, such a system will include a current limit for I.sub.c, limiting it to a value that does not produce a damaging level of optical output power. As diode 18 ages, its efficiency decreases, producing a lower level of optical output for a given I.sub.c. This reduces the amplifier performance and reduces its useful lifetime. In one aspect of the present invention, however, it is realized that I.sub.c may be increased as long as the optical output is limited below the damage level. This increase of I.sub.c allows some compensation for the effect of ageing. This compensation extends the useful lifetime of diode 18, and thus the amplifier system.
It is therefore an object of the present invention to provide a system and technique for extending the useful lifetime of the laser diode by allowing the drive current to increase without exceeding its optical power damage level.
It is a further object of the present invention to provide a system and technique for analyzing the efficiency of a laser diode in an optical amplifier configuration.
It is a further object of the present invention to provide a system and technique for providing a warning as the efficiency of a laser diode in an optical amplifier configuration degrades.
It is a further object of the present invention to provide an amplifier configuration which maintains a substantially constant light output irrespective of fluctuations in light input and/or aging of the amplifier pump laser diode.
It is a further object of the present invention to provide a system and technique having an optical amplifier configuration with improved reliability and predictability.
Still further objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.